Generally, the load on a large-scale motor vehicle is measured using a load measuring device installed on a road. This device, however, is large in size and is high in installing cost so that the installing place and number of devices are limited. Only a partial number of all vehicles can be subjected to measurement. Thus, such a device was insufficient to detect excessive loading.
For the reason described above, in recent years, a load measuring device has been mounted on a vehicle itself. For example, a "self-weight meter" has been proposed. In the self-weight meter, with a sensing element such as a magnetostrictive gauge sensor welded to the surface of an axle shaft (or axle case), distortion generated in an axle when the load on a load-carrying platform is applied to the axle is detected by the sensing element to measure the carrying load for each vehicle.
However, the self-weight meter of such a type may provide a varying detected value. This is because in accordance with the state of a vehicle placed, e.g. where the road face is poor and hence the axles are slanted, loads applied to the axles are in different orientations, and the sensing elements provide different distortions.
Generally, where the rear wheel is attached in a longitudinal direction of a large-scale vehicle, the axle cases are arranged in two rows in the longitudinal direction. Therefore, as shown in FIG. 5A, a leaf spring 13 is positioned so that its both ends are located on the corresponding upper sides 12 (12a and 12b) and its center is secured on a load carrying frame (not shown) of a vehicle body.
In this case, the curving degree of the leaf spring 13 varies with the weight carried on a vehicle so that the relative distance between the axle cases 12a and 12b varies. Therefore, slide plates 5 (5a, 5b) each having a hemisphere in section are arranged on the upper surface of the axle cases 12a and 12b, respectively so that the ends of the leaf spring 13 can slide on these slide plates 5a and 5b.
FIG. 5B shows a trunion-suspension of a rear wheel two-shaft of a large scale vehicle. Both ends of the leaf spring 13 are in contact with the axles 11 through the corresponding slide plate 5 and spacer 10, respectively. The details are shown in FIG. 8.
Taking the above structure in consideration, the applicant proposed the following attaching structure of the load measuring apparatus for a vehicle disclosed in Japanese Patent Appln. No. 6-28037 and shown in FIG. 6.
FIG. 6A is a bottom view of the slide plate 5 for attaching a sensing element in which a base plate 6 is attached. FIG. 6B is a sectional view viewed from the side, in which the sensing element 1 is attached on the one side and covered with a cover 3 from above. FIG. 6C is a sectional view viewed from the front. Incidentally, in this embodiment, the slide plate is made of carbon steel.
As seen from these figures, at both inner ends of the slide plate 5, installing places 5f for sensing elements 1 are provided. Each installing place 5f is located between two screws 5c for attachment of the axle case, and is composed of a rectangular shallow groove 5f1, a narrow deep groove 5f2 formed from the center of the slide plate 5 to that of the shallow groove 5f1 and medium deep grooves 5f3 made on both sides of the deep groove 5f2.
The shallow groove 5f1 is a portion into which a cover described later is to be fit, the medium deep groove 5f3 is a portion into which the sensing element is to be fit, and the deep groove 5f2 is a portion into which a gradient key described later is to be fit.
As seen from FIGS. 6A-6C, a slim rubber 7 is attached to the inside of the side plate 5 from the installing place 5f of the sensing element 1, and a base plate 6 is fit in the center portion of the rubber 7. A groove is formed along the center of the rubber 7. A lead wire 2 of the sensing element 1 is fit in the groove and secured there by adhesive. The tip of the lead wire 2 is connected to the base plate 6.
In the concave portion 5d inside the slide plate 5, a concave protrusion 9a is formed, and a cable 8 is fit in the concave portion. A cover 9b is screwed from above. The cable 8, the end of which is connected to the base plate 6, serves to guide the sum of the detected values by the sensing elements 1, which are attached to both ends of the slide plate 5, to the outside.
An example of the sensing element 1, as shown in FIG. 7A, is composed of a bottom plate 1d and a vertical plate 1e uprighted at its center. The vertical plate 1e is provided with four holes 1e1 and coils 1c combined in a sleeves-tied-back format by using the holes 1e1. A protrusion 1e2 formed from the vertical plate 1e serves to secure the lead wire 2 of the coil 1c.
FIG. 7B shows a gradient key 4 for securing the sensing element 1. FIG. 7C shows a metallic cover covering the upper side of the installing place 5f of the sensing element 1 on the slide plate 5.
The metallic cover 3 has a groove 3a partially formed on its inside. The end of the vertical plate 1e is fit in the groove 3a. The cover 3 is welded or bonded to cover the installing place 5f of the sensing element 1 of the slide plate 5. The sensing element 1 is fit in the cover 3. The gradient key 4, as shown in FIG. 7B, is inserted into the lower side of the sensing element 1 so that it is secured to abut on the bottom of the sensing element 1.
In the structure described above, the installing place 5f of the sensing element 1 is located between the screw holes 5c of the slide plate 5, and the sensing element 1 is arranged and secured using the cover 3 and gradient key 4. These components are complicated in machining and high in production cost.
The sensing element 1 is attached by press-fitting of the gradient key 4. The management of the press-fitting force, however, is so difficult that a small press-fitting force provides an insufficient sensor characteristic and also a too large press-fitting force provides a plastic change in the sensing element 1 leading to insufficient sensor characteristic.
In the load measuring device in the case of a trunion suspension as shown in FIG. 5B, shown in FIG. 8, the slide plate 5 with the sensing element 1 mounted therein is secured to the axle 11 through a spacer 10. Therefore, a component of the spacer 10 must be provided in addition to the sensing element 1, cover 3 and gradient key 4. Specifically, where the slide plate 5 is to be secured to the axle 11, in order that the slide plate 5 can transmit the correct load from an uneven contact face between the slide plate 5 and the axle 11, the spacer 10 for canceling the unevenness is required.
The present invention has been accomplished in view of the above defects, and intends to provide a structure for attaching a sensing element for load measurement of a motor vehicle which can reduce the number of components and provide good sensitivity.